SSC00-I-4 Refaat El-Bordany 14th AIAA/USU Conference on Small Satellites 1 In-Orbit Estimation of the Inertia Matrix and Thruster Parameters of UoSAT-12

This paper describes a Recursive Least Square (RLS) procedure for use in orbit to estimate the inertia matrix (moments and products of inertia parameters) of a satellite. To facilitate this, one attitude axis is disturbed using a reaction wheel whilst the other two axes are controlled to keep their respective angular rates small. Within a fraction of an orbit three components of the inertia matrix can be accurately determined. This procedure is then repeated for the other two axes to obtain all nine elements of the inertia matrix. The procedure is designed to prevent the build up of momentum in the reaction wheels whilst keeping the attitude disturbance to the satellite within acceptable limits. It can also overcome potential errors introduced by unmodeled external disturbance torques and attitude sensor noise. The results of simulations are presented to demonstrate the performance of the technique. The paper also describes an RLS procedure which can be used to estimate the thruster coefficients for thrust levels and alignment of the cold gas thrusters used for attitude control on UoSAT-12. A general on-line method is presented which uses a three axis reaction wheel system to accurately determine the relationship between the commanded and actual torque produced by the thrusters. The calibration algorithm is designed to be robust against external disturbance torques, inertia matrix modelling errors and attitude sensor noise. The results of both simulations and successful in-orbit tests are presented, illustrating the effectiveness of the technique. I